This invention relates in general to composite structures and more particularly to woven composite structures having integrally formed stiffeners as well as to a process and apparatus for making the composite structure.
Composite structures composed of weaves impregnated with suitable resins are particularly useful in the aerospace industry due to their light weight, high strength and ability to withstand high temperatures. The typical composite structure consists of a three-dimensional weave, that is a weave having substantial thickness, and a thermo-setting resin matrix in which the woven strands are embedded. In the usual procedure, suitable yarns are woven together to form the three-dimensional weave, and then the weave is immersed in liquid resin. The resin impregnates the weave. Thereafter the resin is heated to cure it and the cured resin forms a very rigid matrix which holds the woven yarns in place. U.S. Pat. Nos. 3,750,714 and 3,875,973 disclose machines for forming three-dimensional weaves of circular cross-section.
In spite of their high strength, composite structures of the type stated have not been widely used in purely structural capacities, primarily due to the difficulty of providing integrally formed stiffeners in such composites. These stiffeners take the form of ribs and of course substantially increase the strength of the overall composite structure without appreciably increasing the weight. Because of this problem composite materials have, to a large measure, been confined to merely protective uses. For example, the noses of high performance re-entry missiles have a beryllium structural assembly of generally conical shape and a heat shield of composite material extended over the outer surface of the beryllium structure. The beryllium on its inwardly presented surface is machined out to provide ring stiffeners and even possibly longitudinal stiffeners. Beryllium, however, is an extremely expensive metal, and is not easily machined. Furthermore, it is quite toxic when reduced to a dust as occurs during machining.